1. Technical Field
The invention concerns a method for the regulated operation of a regeneratively heated industrial furnace, in particular, with a melting tank, in particular for glass, as well as a control device that is formulated for the execution of the method. The invention also concerns an industrial furnace.
2. Description of the Related Art
In principle, an industrial furnace is not restricted to the use in the production of glass. For example, an industrial furnace of the type named in the introduction can also be used in the production of metal or similar products. A regenerative industrial furnace of the type named in the introduction has, however, proven itself to be particularly suitable in glass production for the melting of glass.
Thus far, the control of the regenerative glass melting furnace—that is, regularly by means of control via the upper furnace as a control path—has been entrusted exclusively to PID controllers, the goal of which is the control of an upper furnace temperature, and the output of which represents either a quantity of fuel itself or else a quantity of combustion air which the quantity of fuel then follows in an adjustable relationship.
What is problematic with this is that in actuality, such temperature regulators regularly prove themselves to be unsuitable for regulating the temperature of a regenerative glass melting furnace in a successful, stable manner, and to that extent, they remain unused. The first reason lies in the design of the control that has been followed thus far, which has an inherent systematic tendency to expand slight temperature differences between the regenerators further and further. Within that context, the use of fuel between the flame sides also increases further and further, without a nominal value of the furnace temperature ever being reachable, that is, the control does not converge upon the nominal value of a furnace temperature.
From DE 36 103 65 A1, a method is known for the technologically controlled regulating of an upper furnace heater of an industrial furnace with which a fuel stream is provided for the control of a vault temperature of the upper furnace, and the problem of a regenerative lateral asymmetry is left to subjective influences. It was found that temperature differences in the furnace temperatures between heating on the left and heating on the right have their cause to a predominant degree in the corresponding temperature differences of the associated regenerators. In individual cases, peak temperatures of the left regenerator can be around 45° C. lower than those of the right regenerator, and at the same time, the temperatures in the furnace chamber—that is, regularly in the upper furnace—can be around 20° C. lower with heating on the left than the same temperatures with the heating on the right.
What is desirable is a technological control concept that essentially converges on a nominal value of the furnace temperature and in particular rectifies the problem of the lateral asymmetry through regulation technology.
A second reason lies in the fact that conventional regulation methods incorrectly take for granted that an uncontrolled admission of air is constant, or else they correct the supply of combustion air exclusively on the basis of a measured value of residual oxygen that is measured either manually or continuously which, however, as a result of its non-linear context with the combustion air, cannot achieve any optimal control dynamics. In particular, this approach cannot evaluate the range of substoichiometric heating because from a comparison of different process states each with 0% residual oxygen, no sensible action in terms of control technology can be justified any longer.
A third reason lies in the fact that conventional control methods do not take into consideration the special requirements of regenerative heating with which the combustion air, in addition to its function as an oxygen carrier for the combustion, likewise serves as a transport medium for the heat from the regenerator into the furnace chamber. Therefore, for the compensation in terms of control technology of uncontrolled entry of false air, it makes a difference whether the uncontrolled false air that is to be compensated for has entered before the regenerator—and therefore participates in the heat transport—or whether it only enters after the regenerator—and thus only heats the regenerator on one side on the exhaust gas side with additional exhaust gas heat, and therefore disrupts the thermal symmetry of the regenerators.
The so-called “ratio control” between the fuel stream and the combustion air stream is widespread and common in practice. Within that context, either the quantity of combustion air follows the fuel in an adjustable relationship or, conversely, the fuel stream follows the quantity of combustion air in an adjustable relationship.
In that regard, the ratio specifications are empirically set in such a way that in the exhaust gas stream, a residual oxygen value is set that is estimated as optimal.
What is problematic with this is at minimum that uncontrolled entries to or escapes from the combustion air stream are taken for granted as either not present or else as constant. As has been explained above, though, this condition is not present.
Rather, in regeneratively heated glass furnaces, typically up to 10% of all of the air that arrives for combustion penetrates in an uncontrolled manner into the regenerators or the furnace chamber as false air, whereby the supply of uncontrolled false air is in no way constant, but rather is influenced by the furnace pressure, temperatures, and other operating parameters. Losses of uncontrolled false air are likewise observed, for example by the short circuit flow of combustion air directly in the exhaust gas stream in the case of a leaky reversing blade.
Such deficiencies cannot be compensated for by so-called “cross limit ratio controls” with which the combustion air follows the maximum of the nominal value and the process value of the fuel in an adjustable relationship and, conversely, the fuel is limited if less air is available than would correspond to the air ratio that was set. Because this method also tacitly and incorrectly presupposes that uncontrolled false air is either not present or is supplied constantly.
One known approach to the solution of the problem is provided by a method indicated as “oxygen trimming” with which the empirical correction of the air ratio is replaced by an automatic correction which is oriented to the difference between a target value and a continuously measured value for the residual oxygen content in the exhaust gas.
However, this method can only be used in the case of an oxidizing firing, that is, as long as sufficient residual oxygen is still available. The method of “oxygen trimming” is tied to the disadvantage that a non-linear relationship exists between the residual oxygen value in the exhaust gas and the associated value of the combustion air stream which considerably affects the dynamics of such a regulation.
The method known as “lambda control” is known from the control of internal combustion engines, in particular in automotive engineering, with which the mixture of fuel and air is automatically corrected in such a way that the nominal value for the measuring result of a lambda probe in the exhaust gas is carried out downstream from the engine.
An improved control concept for an industrial furnace of the type mentioned in the introduction is desirable in consideration of the long reaction times that are typical for the regenerative heating of industrial furnaces between a change in the air ratio and the measurable change as a result in the measured value of an exhaust gas analysis measurement. In the end, this is caused as a result of the significantly higher volume of the regenerator and the furnace chamber in comparison with an internal combustion engine. Therefore, an insufficiently simple transfer of a method from automotive engineering to an industrial furnace runs into considerable problems in the control dynamics. In particular, neither the method of “oxygen trimming” nor the “lambda control” that is known from automotive engineering take into consideration the periodicity of regenerative heating or firing with a regenerative industrial furnace of the type mentioned in the introduction. Instead of that, typical recurring trend patterns of the uncontrolled entry of false air or of the uncontrolled false air escape can be observed with a regeneratively fired industrial furnace which cannot be compensated for or can only be incompletely compensated for by a slow, gradual correction of the air ratio, while a fast correction of the air ratio fails because of unfavorable control dynamics.